Described herein are discrete, spaced, large area photovoltaic cells which are typically, but not necessarily, divided into a matrix of electrically-isolated, small area segments. The spacing and division of the large area cells is provided by an electrically-insulating material applied directly atop the semiconductor body of semiconductor material for separating adjacent large area cells and/or adjacent small area segments thereof. Also disclosed as a principal feature of the present invention is a process for fabricating, either by continuous or batch processing (1) the large area semiconductor cells from a continuous strip of photovoltaic material, and (2) the small area segments of the large area cells.
Although crystal silicon devices are the basis of the huge semiconductor industry, devices made from crystal silicon have fixed parameters which are not variable as desired, require large amounts of material, are only producible in relatively small areas and are expensive and time consuming to produce. Devices based upon amorphous silicon can eliminate these crystal silicon disadvantages. Amorphous silicon has an optical absorption edge having properties similar to a direct gap semiconductor and only a material thickness of one micron or less is necessary to absorb the same amount of sunlight as the 50 micron thick crystalline silicon. Further, amorphous silicon can be made faster and more easily in larger areas than can crystalline silicon.
Accordingly, a considerable effort has been made to develop processes for readily depositing amorphous semiconductor alloys or films, each of which can encompass relatively large areas, if desired, limited only by the size of the deposition equipment, and which could be readily doped to form p-type and n-type materials when p-n junction devices are to be made therefrom equivalent to those produced by their crystalline counterparts.
Greatly improved amorphous silicon alloys having significantly reduced concentrations of localized states in the energy gaps thereof and high quality electronic properties have been prepared by glow discharge deposition, as fully described in U.S. Pat. No. 4,226,898, Amorphous Semiconductors Equivalent to Crystalline Semiconductors, issued in the names of Stanford R. Ovshinsky and Arun Madan on Oct. 7, 1980, and by vapor deposition as fully described in U.S. Pat. No. 4,217,374, issued in the names of Stanford R. Ovshinsky and Masatsugu Izu, on Aug. 12, 1980, under the same title. As disclosed in these patents, which are incorporated herein by reference, fluorine is introduced into the amorphous silicon semiconductor to substantially reduce the density of localized states therein. It is believed that the activated fluorine readily diffuses into and bonds to the amorphous silicon in the amorphous body to substantially decrease the density of localized defect states therein, because the small size of the fluorine atoms enables them to be readily introduced into the amorphous body. The fluorine bonds to the dangling bonds of the silicon and forms what is believed to be a partially ionic stable bond with flexible bonding angles, which results in a more stable and more efficient compensation or alteration than is formed by hydrogen and other compensating or altering agents. Fluorine is considered to be a more efficient compensating or altering element than hydrogen when employed alone or with hydrogen because of its exceedingly small size, high reactivity, specificity in chemical bonding, and highest electronegativity.
It is known that the efficiency of a photovoltaic device may be enhanced by stacking cells atop of each other. This concept of utilizing multiple cells, to enhance photovoltaic device efficiency, was discussed at least as early as 1955 by E. D. Jackson in U.S. Pat. No. 2,949,498, issued Aug. 16, 1960. The multiple cell structures therein discussed utilized p-n junction crystalline semiconductor devices. Essentially, the concept is directed to utilizing different band gap devices to more efficiently collect various portions of the solar spectrum and to increase open circuit voltage (Voc.). The tandem cell device has two or more cells with the light directed serially through each cell, with a large band gap material followed by a smaller band gap material to absorb the light passed through the first cell or layer. By substantially matching the generated currents from each cell, the overall open circuit voltage of each cell may be added, thereby producing a device which makes full use of the energy produced by incoming light.
It is of obvious commercial importance to be able to mass produce photovoltaic devices. Unlike crystalline silicon, which is limited to batch processing for the manufacture of solar cells, amorphous silicon alloys can be deposited in multiple layers over relatively large area substrates to form solar cells in a high volume, continuous processing system. Continuous processing systems of this kind are disclosed, for example, in pending patent applications: Ser. No. 151,301, filed May 19, 1980, for A Method of Making P-Doped Silicon Films and Devices Made Therefrom, now U.S. Pat. No. 4,400,409; Ser. No. 244,386, filed Mar. 16, 1981, for Continuous Systems For Depositing Amorphous Semiconductor Material; Ser. No. 240,493, filed Mar. 16, 1981 for Continuous Amorphous Solar Cell Production System; Ser. No. 306,146, filed Sept. 28, 1981 for Multiple Chamber Deposition and Isolation System and Method; and Ser. No. 359,825, filed Mar. 19, 1982 for Method and Apparatus for Continuously Producing Tandem Photovoltaic Cells. As disclosed in these applications, a substrate may be continuously advanced through successive triads of deposition chambers, wherein each chamber is dedicated to the deposition of a specific material. In making a solar cell of p-i-n type configuration, the first chamber of each triad is dedicated for depositing a p-type amorphous semiconductor material, the second chamber of each triad is dedicated for depositing an intrinsic semiconductor material, and the third chamber of each triad is dedicated for depositing an n-type amorphous semiconductor material.
The resultant roll of large area photovoltaic cells manufactured by the mass production glow discharge deposition technique described hereinabove, comprises an elongated strip of substrate upon which successive semiconductor layers are deposited. For purposes of the instant application, the strip of substrate material (regardless of length) with semiconductor layers deposited thereonto will be referred to as the "semiconductor material".
It is well known that following the deposition of these semiconductor layers, a further processing step is needed to fabricate an operable semiconductor device. In this step, the semiconductor material must be provided with an anti-reflective, transparent, high light-transmissivity, high electrical-conductivity coating (hereinafter referred to as the "transparent conductive coating"). It is preferred that this transparent conductive coating be applied to the exposed surface of the upper semiconductor layer so as to form a matrix of electrically-isolated, small area segments which can be electrically-interconnected to form a large area photovoltaic cell.
It is desirable, based on present technology, to divide the large area photovoltaic cell into the matrix of electrically-isolated segments, described hereinabove, to obtain the greatest electrical output from the large area cell. In a large area semiconductor device, there is a likelihood that defects in the semiconductor layers, such as shorts, will occur. Depending on the location and the severity of the defect, the electrical output of the entire semiconductor body may be significantly decreased. For these reasons, it is advantageous to (1) divide the large area semiconductor device into the plurality of electrically isolated, small area segments, (2) identify those segments not performing up to a preselected set of output specifications, and (3) electrically connect only those small area segments of the large area photovoltaic cell that meet those preselected specifications. Note, however, technology is progressing to the point that shorts may not be a problem in the foreseeable future. Accordingly, the present invention is not limited in use to applications in which the large area cell is divided into small area segments, but is also applicable for uses as detailed hereinafter.
In prior art large area photovoltaic cells, the transparent, conductive coating was deposited as a continuous layer, which was subsequently scribed to form the electrically-isolated small area segments. This was a multi-step process involving (1) the deposition of the transparent, conductive coating, (2) the application of a patterned resist material by techniques such as photolithography, or the like, and, subsequently, (3) an etching step in which wet chemistry, or in some cases, plasma-etch techniques were employed. These prior art scribing processes were time consuming, labor-intensive, and involved wet chemistry. In another prior art method, the transparent, conductive coating was deposited in a discontinuous pattern by the use of a mask or stencil. While this technique offers the advantages of being a dry process suitable for a continuous mode of production, implementation of this process has inherent drawbacks; namely, problems involving the alignment of the deposition mask with the photovoltaic material.
In contrast to the foregoing, the present invention, inter alia provides a large area photovoltaic cell on which a screen printed, electrically-insulating pattern defines the matrix of electrically-isolated, small area segments. Because the scribing is accomplished by a dry, screen printing process in which the small area segments are clearly defined, the present invention represents a substantial improvement over prior art (1) photovoltaic cells and (2) scribing or discontinuous transparent, conductive coating deposition techniques. In the instant invention, the electrically-insulating material may define the matrix of small area segments of the large area cell, thereby eliminating the need to scribe the transparent, conductive layer. Obviously, the elimination of the etching step eliminates a method which could (1) impair the electrical performance of the semiconductor layers underlying the transparent, conductive coating; (2) remove too much of the transparent, conductive coating, thereby reducing current collection; or (3) remove an insufficient amount of the transparent, conductive coating, thereby having adjacent small area segments remain electrically connected.
Furthermore, as detailed supra, the photovoltaic cells of the instant invention are well adapted for continuous production in a cost efficient manner. In the preferred embodiment of this invention, the large area cells are fabricated from the roll of semiconductor material. Since this roll of semiconductor material is typically 1000 feet in length, it should be apparent that it is necessary to cut the roll to form individual large area cells therefrom. In order to efficiently cut large area cells from that 1000 foot length without creating short circuits, prior art methods would first deposit a transparent, conductive coating onto the surface of the semiconductor material and then remove the coating between adjacent large area cells. In contrast thereto, the present invention enables the large area cells to be cut without creating shorts by depositing the electrically-insulating material between the adjacent large area cells. This results in an obvious increase in efficiency of operation and economy of costs.
Note that, although a photovoltaic cell having an amorphous semiconductor body including fluorine has been described hereinabove, the present invention is not limited to amorphous semiconductors fabricated from specific process gases. This application is equally adapted for use with photovoltaic cells of any composition, whether (1) amorphous, crystalline or polycrystalline; or (2) including fluorine. Moreover, U.S. patent application Ser. No. 422,155 filed Sept. 23, 1982, entitled Compositionally Varied Materials and Method for Synthesizing The Materials and assigned to the same assignee as the instant patent application, provides a basis for obtaining photovoltaic quality response from materials previously tried and discarded, or synthesized new materials.
The many objects and advantages of the present invention will become clear from the drawings, the detailed description of the invention and the claims which follow.